The present invention is directed to a closure for use in conjunction with open ended medical implants, such as bone screws, hooks and the like that are used in certain types of spinal surgery. The purpose for the closure is to capture a rod member within the head of the implant and to also lock the captured rod member in position relative to the implant so that the two do not move rotationally or axially with respect to one another.
Medical implants such as bone screws and hooks are used in many types of spinal surgery wherein a structure is built about and within the spine to provide support or strength to diseased, missing or damaged spinal elements of the patient. The implanted apparatus includes a number of different parts which vary with each patient and that are linked together to form a stable support system. For example, bone screws are typically threadably mounted in the bones and have heads that receive rods or similarly shaped connectors that join other bone screws or other elements of the system together.
Bone screws of this type may have either closed heads wherein a rod is threaded into the head from the side or an open head wherein a rod is laid in a channel formed in the head. The later type of bone screws are referred to as open ended bone screws. Such open ended bone screws are favored in many types of spinal surgery, since a rod does not have to be threaded through the bone screw which is difficult to do in the tight space provided and because the spine curves making insertion of a rod that follows the spinal curvature very difficult. On the other hand, the open ended bone screws allow the rod to be laid or drawn into the open channel of the head which greatly simplifies installation in comparison to threading the rod through a head of the bone screw.
While open ended medical implants such as bone screws, hooks and the like are often easier to use in comparison to closed end implants of the same type, the open ended implants do have associated problems. In particular, in open headed implant systems it is extremely important that there be no slippage or relative movement between the connecting rods and the implants, such as bone screws, to which the rods are joined. The rods in this case are usually not linear, but are curved to follow the curvature of the spine. If the rod is allowed to slip either axially or rotate within the bone screw or other connector to which the rod is secured, at a minimum the effectiveness of the implant is decreased and it is possible that the patient could be severely injured. Consequently, it is extremely important to secure the rod to the bone screw or other connector so that no slippage occurs. In some of the prior art devices, open headed implants, which typically have a pair of spaced arms, have been internally threaded and have received a threaded plug between the arms, which plug is designed to abut against the rod under torque and both capture the rod in the head and lock the rod into place.
The plug of the prior art has an outer cylindrical surface that is threaded and is received in the threads of the arms. In order to do this the plug has a comparatively large diameter so that the plug extends out sideways relative to the bone screw or the like or the bone screw must be made wider than is necessary. That is, the plug makes the overall implant wider than is necessary. This presents problems to the surgeon installing the system as space is very limited along the rods in many situations and there is not enough room for all of the total system structure to attach to the rods or, if there is sufficient room for attachment of elements, then there may be insufficient room for the surgeon to manipulate the rod by use of benders to shape the rod to conform to the proper spinal curvature, as is required in many of the surgeries using these devices. It is also noted that other prior art devices have a ring that goes around the outside of the arms. Such a ring is very bulky, taking up a great amount of space along the rods to which the implant is joined. Consequently, it is very desirable to eliminate this side to side extension of the closure along the axis of the rod. That is, it is desirable that the implant and closure both have a low side to side profile.
Secondly, as has been mentioned above, the rods are typically bent throughout the length thereof, such that the rods follow the curvature of the spine. Because of this, the rods seat in the open head of the bone screws and other implants in such a manner that the rod has curvature associated with it, even within the head. When plugs of the prior art seat against the rod, typically the rod is bowed so that the rod engages the plug only at the radially outer edges thereof on both sides. The plug is then torqued against the rod to tighten it down against the seat in the bone screw channel, but it is quite difficult to tighten the rod sufficiently so that the portion of the rod within the head has no curvature after tightening. Subsequently, during use of the device, the rod may be flexed or bent by activity of the person, especially greater activity than normal, such as occurs in an accident or the like, and the remaining curvature of the rod flexes somewhat, thereby loosening the plug. When this occurs, the rod may slip axially or rotate relative to the implant and/or the plug may become loosened and work out of the implant. Both of these situations are very undesirable.
Therefore, there is a need to provide a closure for bone screws and other medical implants having an open head such that the closure has both a relatively thin profile along the axis of the rod or the like and also where the closure has a mechanism whereby the rod can be locked centrally opposite the seat to prevent the rod from becoming loosened should the rod flex during usage.
A closure is provided for use in conjunction with open headed medical implants, such as bone screws and hooks, which receive rods and other structure for interconnecting with various parts of an overall system. The closure is lozenge shaped with opposed cylindrical sectors that are radially outwardly threaded and that are joined by generally planar surfaces. The threads on the cylindrical sectors are sized and shaped to mate with the threads on internal surfaces of arms of the implants. The closure also includes a central passthrough threaded bore that is coaxially located with respect to the closure.
A set screw having a base with a radially outward threaded surface and a head is threadably mounted in the bore during installation of the closure. The set screw includes a point that extends from the closure bore when fully installed so as to engage a rod member located in the head of the implant. The set screw may be of a break-off head variety or may retain its head after installation. Also the set screw may be designed to rotate with the closure during installation or independently of the closure. In one embodiment the set screw is designed to extend entirely through the bore of the closure during installation and engage a rod in an implant head under torque until the head of the set screw breaks away at a predetermined torque. In such an embodiment the closure functions to capture the rod in the head and the set screw functions to lock the rod in place relative to the head of the implant.
A set of tools is also provided for installation of the closure since the closure has discontinuous threads thereon. The installation tool includes a handle for rotating and a head that has a channel for receiving the closure. The head also has a pair of wings on either side of the channel which are radially outward threaded and positioned so as to mate with the closure such that the threads on the closure and the threads on the wings form a generally continuous thread that is sufficient to allow the structure to be threadedly mated without cross threading or having thread turns at different levels on the arms mate with a single thread on the closure and that may be utilized advantageously to mount the closure in the head of the implant. A second tool including a shaft with a threaded tip may also be mounted in the bore of the closure during installation to allow the installer to better grip the closure.
Therefore, the objects of the present invention are: to provide an overall medical implant system wherein the system includes various bone screws, hooks and the like with open heads that are joined with a series of rod-like structures by use of a closure in accordance with the present invention; to provide such a closure which is relatively thin in profile along the axes of the rods with which it is utilized; to provide such a closure which is lozenge shaped and includes a pair of cylindrical sectors that are opposed to one another and which are externally threaded so that the closure is only partially threaded and such that the thread is discontinuous between the sectors, but in such a way as to threadably mate with threads on internal surfaces of arms of open headed implants; to provide such a closure which includes a central passthrough and threaded bore; to provide such a closure with such a bore in conjunction with a threaded set screw received in the bore and passing at least partially through the bore during installation; to provide such a closure wherein the closure captures a rod member within a head of an implant and wherein the set screw locks the rod member in place relative to the implant head to prevent rotation or axial relative movement of the rod member in relation to the head; to provide such a closure having a relatively thin profile upon installation; to provide a set of tools to be used in conjunction with installation of the closure; to provide such tools including a closure installation tool having a head that receives the closure and a pair of wings that extend outwardly on either side of the closure with threads on outer surfaces of the wings that compliment threads on the closure so as to provide, when joined, a substantially continuous thread for use in installation of the closure; to provide such an overall system and, especially, a closure that is relatively easy to use, economical to produce and especially well adapted for the intended usage thereof.
Other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention.
The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof.